When log4net stops writing to the log file it can depend on several things. Here are a few hits that may be can help you out.
Logging in mirroring 2 for EPiServer 6Not correct XML file
Try to open your sites EPiServerLog.config as an XML file in Internet Explorer.
Permissions for files
The web server process user has to have write and modify rights in the file system where the log files should be stored. Use
FileMon and filter out you folder for log messages, it should be activity there.
Using log4net before it initialize
Log4net has to initialize before any loggers are created. So in classes created early in the initialization phase (in EPiServer Global.asax.cs) create the logger when used and not in the static constructor. Otherwise log4net want start and no log files are created.